Visco-elastic foams are characterized by a slow and gradual recovery after compression. Such materials are well known in the prior art and are much appreciated because of their energy-absorbing properties. Visco-elastic foam materials are found in a wide variety of application fields for cushioning (for example, in pillows, seat covers, mattresses etc.), as sound- and/or vibration-damping materials or as an impact protection.
Among the visco-elastic foam materials, those made of polyurethanes are certainly of the greatest importance. On the one hand, this is due to the fact that the physical properties of the polyurethane foam to be obtained can be adjusted very exactly by selecting the polyol and isocyanate components employed and optionally other auxiliaries, and on the other hand, it is also because foam materials of almost any shape and structure, which may be very complex, can be prepared by the “in situ” preparation (optionally on location).
During the preparation of polyurethanes, usually two or more liquid streams are combined. The mixing of these liquid streams initiates polymerization and, as the case may be, the foaming of the polymerizing material. The polymerization and shaping are often effected in one step, typically by shaping or spraying the reaction mixture while still in a liquid state. In addition, polyurethanes are also often prepared in the form of slabstock, which is subsequently cut to the desired shape.
In most cases, the above mentioned liquid streams are, on the one hand, a polyfunctional organic isocyanate component (often referred to as “component A”) and, on the other hand, polyfunctional monomers or resins which have an appropriate reactivity towards isocyanates and may optionally contain further auxiliaries. The latter mixture, which is often referred to as “component B”, typically comprises one or more polyol components for the major part thereof.
Now, to obtain a polyurethane foam of a particular composition, the above described liquid streams are dosed correspondingly before being mixed. Usually, foaming is achieved by adding water to component B, which water reacts with the polyisocyanate of component A to form an amine and to release CO2, which in turn functions as a foaming gas. Alternatively or additionally to the use of water, volatile inert organic compounds or inert gases are often used.
The majority of conventional polyurethane foams are block copolymers comprising spatially separated regions of different phases with high and low glass transition temperatures (TG). The glass transition temperature separates the brittle energy-elastic range (=glass range) below from the soft entropy-elastic range (=rubber-elastic range) above. These high and low glass transition temperatures of different phases within the polymer normally set limits to the temperature range within which the material can be used. The DMA (“dynamic mechanical analysis”) spectra of such materials are usually characterized by a relatively flat region (“modulus plateau”) between the different glass transitions.
The phase of low glass transition temperature in such materials is usually (though not always) derived from a “block” of low glass transition temperature, which is formed first and subjected to polymerization only subsequently. In contrast, the phase of high glass transition temperature normally forms only during the polymerization due to the formation of urethane moieties which occurs then. The block of low glass transition temperature (often also referred to as “soft block”) is usually derived from a liquid or from an oligomeric resin of low melting temperature that contain a large number of groups reactive towards isocyanate moieties. Polyether polyols and polyester polyols are examples of such oligomeric resins.
In conventional polyurethanes, the hard (high glass transition temperature) and soft (low glass transition temperature) phases arrange towards one another during polymerization and subsequently separate spontaneously to form morphologically different phases within the “bulk polymer”. Accordingly, such materials are also referred to as “phase-separated” materials.
In this context, visco-elastic polyurethanes are a special case in a way, namely in which the above described phase separation occurs incompletely or not at all.
To be distinguished from such a “structural visco-elasticity” in polyurethane foams with (predominantly) open cells is a visco-elasticity that is due to a pneumatic effect. Namely, in the latter case, almost closed cells, i.e., cells with little opening, are within the foam material. Because of the small size of the openings, air will re-enter slowly after compression, which results in a slowed-down recovery.
Examples of such a visco-elastic foam based on a pneumatic effect are the commercially available products Cosypur® and Elastoflex® of the Elastogran GmbH.
In the prior art, many methods have been described for the synthesis of polyurethane foams with structural visco-elasticity, which methods mostly share the use of a special polyether polyol composition in addition to an isocyanate component that is more or less freely selectable.
Such polyether polyols are usually the product of the polymerization of epoxides, such as ethylene oxide (EO), propylene oxide (PO), butylene oxide, styrene oxide or epichlorohydrin, with themselves or by addition of such epoxides, optionally in admixture or sequentially, to starting components with reactive hydrogen atoms, such as water, alcohols, ammonia or amines. Such “starter molecules” usually have a functionality of from 1 to 6. Depending on the process control, such polyether polyols may be homopolymers, block copolymers, random copolymers, capped polymers or polymers tipped with a mixture of different epoxides. To specify such polyether polyols, various characteristics have become established in the prior art:    i.) hydroxyl functionality, which depends on the starter molecule starting from which the polyether polyol is synthesized;    ii.) hydroxyl or OH number, which is a measure of the content of hydroxyl groups stated in mg of KOH/g;    iii.) when epoxides in which the ring opening causes the formation of different (i.e., primary or secondary) hydroxyl groups are used, on the one hand, the proportion of the respective epoxides in the polyether polyol is stated, and on the other hand, the proportion of primary or secondary hydroxyl groups based on the total number of hydroxyl groups present in the polyether polyol is stated;    iv.) the molecular weight (Mn or Mw), which is a measure of the length of the polyalkylene chains of the polyether polyols.
The above mentioned quantities can be related to one another through the following equation: 56,100=OH number·(Mw/hydroxyl functionality).
Examples of the use of polyether polyol compositions in polyurethane synthesis are found, for example, in WO 01/32736 A1, WO 02/088211 A1, WO 02/077056 A1, WO 01/25305 A1, U.S. Pat. No. 5,420,170, U.S. Pat. No. 6,653,363 B1 and U.S. Pat. No. 6,136,879 A.
A drawback of the examples stated above, which (almost) exclusively use polyether polyols as the B component, is the fact that a large amount of fossil raw materials must be provided for the synthesis thereof, and consequently, they cause a very high CO2 emission (on the one hand, the epoxides are ultimately produced from compounds obtainable from petrol, mainly ethene and propene; on the other hand, a large amount of fossil raw materials is combusted for reacting petrol into the required intermediates ethene and propene).
Thus, under the aspect of renewability, a complete or at least partial replacement of the synthetic polyether polyols by substantially more readily accessible compounds and especially by renewable raw materials would be desirable. Approaches to achieving this object are found, for example, in EP 0826706 A2, DBP 1113810, DE 3708961 C2, DE 3316652 C2, U.S. Pat. No. 4,839,397 and US 2006/0270747 A1, which mainly teach the use of castor oil as a renewable raw material for the preparation of various polyurethane systems.
This concept gradually seems to enter the field of visco-elastic polyurethane foams as well, as shown in WO 2007/085548 A1. The invention described therein relates to a process for the preparation of open-pore visco-elastic polyurethane flexible foams based on renewable raw materials by reacting:                a) polyisocyanates with        b) a polyol mixture consisting of        bi) compounds having at least two isocyanate-reactive hydrogens and an OH number of 20 to 100 mg of KOH/g; and        bii) compounds having at least two isocyanate-reactive hydrogens and an OH number of 100 to 800 mg of KOH/g; and        biii) compounds having at least one and at most two isocyanate-reactive hydrogens and an OH number of 100 to 800 mg of KOH/g; and        c) foaming agents;characterized in that each of components bi) and bii) contains at least one compound which contains renewable raw materials or their reaction products.        
Castor oil is more preferably employed as compound bii). A drawback of this process is the fact that the main component, i.e. bi), is a reaction product of a renewable raw material with epoxides, i.e., is also a polyether polyol ultimately; in particular, a chemically unaltered renewable raw material cannot be exclusively employed here.
Therefore, it is the object of the present invention to provide a polyether polyol composition containing as high as possible a proportion of a chemically (almost) unmodified renewable raw material, which can be used to prepare polyurethane foams of high visco-elasticity.